1. Field of the Invention
The present disclosure relates generally to high-energy radiation monitoring and detection; more particularly, it relates to an ion chamber for detecting high-energy radiation.
2. Background Art
Detectors of high-energy, ionizing radiation are used in various applications. Such detectors, for example, include ion chambers, proportional counters, Geiger-Mueller counters, and scintillation counters. Among these, ion chambers are commonly used in neutron detectors. FIG. 1 shows a basic system for neutron detection that includes a target chamber 13, an ion chamber 14, and electronics. Fast neutrons 12 are produced by a neutron source 11. These fast neutrons 12 interact with hydrogen nuclei in the target chamber 13 until their velocity is reduced to the average thermal velocity of the target. The thermal (slow) neutrons are then scattered from the target 13 to the ion chamber 14.
In a typical neutron detector, the ion camber 14 is filled with a gas (such as He-3) that can interact with the thermal neutrons to produce ions. When a He-3 atom absorbs (captures) a thermal neutron, a nuclear reaction occurs and the resultant products are fast-moving tritium (H-3) and a proton. These fast-moving particles travel through the gas, ionizing some of the remaining He-3 atoms and thus creating an equal number of positive and negative ions. When a voltage is applied across the electrodes 15, 16 in the ion chamber 14, an electric field is created in the space between the electrodes. The ions move in response to the electric field with the positive and negative ions pulled in opposite directions toward each electrode. The ions are eventually neutralized at electrodes 15 and 16 resulting in an ion current that is directly proportional to the number of ions transferred to the electrodes. The number of ions transferred to the electrodes depends on their formation rate and hence the neutron flux. Thus, the ion currents measured by the ion chamber may be used to derive the magnitude of the neutron flux.
The ion current-voltage (I-V) characteristic curve for a gas in an ionization detector usually contains a flat region, called the “region of saturation” or “plateau.” Shown in FIG. 3 is an example of an ion I-V characteristic curve for an ion chamber filled with He-3. As seen in FIG. 3, in the plateau region, the ion current is insensitive to the voltage applied to the electrodes. Therefore, it is advantageous to operate the detector in this region because any noise in the applied voltage is minimally coupled to the ion current. The exact voltages at the beginning and end of the plateau region depend on the specific chamber geometry, but for a typical ionization detector employing He-3, the plateau begins around 10V. Operation of these detectors below 10V leads to increased noise and weak ion current, and therefore such detectors require high operating voltages (greater than 10V in the example shown in FIG. 3). Other (heavier) gases requires much higher voltage to reach plateau, usually 400, 600V.
However, in general, gases employed in ionization detectors need not have a plateau in their current-voltage characteristics. For example, the relative scarcity of He-3 has lead to the use of gases such as BF3, which is known not to have a plateau in its I-V characteristic curve, as shown in FIG. 4. Rather, the ion current steadily increases with increasing voltage. As a result, ionization detectors based on BF3 are more susceptible to noise than their He-3 counterparts.
As is the case with all known ionization detectors, the ion current generated by the ionizing radiation is extremely small (on the order of 10−12 A), making it very difficult to accurately determine neutron flux. In addition, temperature and humidity changes in various electronic components, cables, etc. may further compromise the accuracy of the measurements. The situation is even worse under field conditions, which often include wide variations in temperature and humidity in addition to unwanted sources of vibration.
Furthermore, instability in leakage currents may also significantly degrade the accuracy of repeat measurements. Leakage current is a current through the detector system that is not due to ion transport through the ion chamber 14. Leakage currents may be due to cables, connections, parasitic current in the components, moisture contamination of the amplifier circuit or other components, or any number of other factors. Thus, leakage current depends on a highly convoluted function of temperature, humidity, age of components, and any number of other factors. Because the ion current in an ion chamber is on the order of 10−12 A or less, leakage current may be a significant fraction of the total measured current, and any variation in the leakage current may significantly impact the accuracy of the measurements.
In a setup described above and shown in FIG. 1 and FIG. 5, the ion chamber (or counter) is maintained at an equilibrium high voltage V0 (i.e., constant voltage mode) so that it is ready to detect constant flux of high-energy radiation (e.g., the ion chamber for detecting neutron flux shown in FIG. 1) or pulses of high-energy radiation (e.g., proportional counters or Geiger-Mueller counters for detecting gamma rays). In addition, modern chambers operate in a pulsed high voltage mode, in which the voltage applied across the electrodes in the ion chamber is pulsed. That is, the ion chamber is not maintained at an equilibrium high voltage. Rather, these devices may use pulses of high voltage to monitor or measure neutron flux or other ion generation in a steady state.
There remains a continuing need for ion chambers that provide more reliable and accurate measurements of high-energy radiation.